Why? Some have been wildly popular.
Budget constraints is a big one, it would limit everything from special effects to wardrobe to talent (both in front of and behind the camera).
But I think the biggest factor would be lack of vision. Arguably the most important man involved in the Avengers franchise and its supporting films is Kevin Feige. The man a had vision and a plan to execute it. Without him we would not be having this conversation. TV while similar on the surface is a whole different animal underneath. Decisions are made by committee's and big wigs who are not in it for the story or the characters but the advertising and the profit margin. I understand that this also applies to movies as well but the big difference is control. When a movie company signs off on a project it is handed over to the producer who is more or less the boss from that point on. Again that producer still has to answer to his higher ups, but when it comes to the Marvel productions I think Kevin is given a lot more leeway than others mainly because they understand how important that control is. One of the reasons that Marvel created its own studio was maintain control over its franchises. In television there would be too many fingers in the pot.
Smallville would be the exception to this rule, but even it in the end faltered. But it did have a decent run. When it comes down to it, I just wouldn't trust TV executives to handle these characters with the respect and care that I would want or expect them to.